In Otto engines operating with fuel injection, the fuel is directly injected into the combustion chamber of the cylinder by a fuel injector. Dependent upon the moment of such injection, essentially two modes of operation may be distinguished.
In the "homogeneous" operational mode the fuel is injected into the combustion chamber at an early moment, generally during the inflow of the combustion air, that is, when the intake valve is open. In this manner, a satisfactory homogenization of the fuel-air mixture is achieved. Such a mode of operation is particularly suitable at high engine loads.
In the "stratified-charge" operational mode the injection occurs only after the intake valve is closed and when the piston, during its upward stroke, is close to its upper dead center. As a result, the fuel is mixed only with one part of the combustion air contained in the cylinder and also, only a locally limited mixing occurs until the fuel is ignited by the ignition device. Such a mode of operation is preferably used in the partial load or idling run of the engine. It is an advantage of this mode of operation that the engine may run without throttling of the intake air and yet, there is no risk that the fuel/air mixture is excessively lean in the vicinity of the ignition device for ensuring a reliable ignition.
For the above-outlined operational modes different processes for introducing the fuel into the cylinder chamber and for forming the mixture have been known. Such processes may be divided into two categories, namely, the "jet-guided" process and the "wall-guided" process.
In the "jet-guided" process the injected fuel jet is aimed directly onto the spark plug. The injected fuel cloud mixes with the combustion air and is ignited by the ignition device. A reliable stratified-charge operation is accordingly ensured only if the spark plug is positioned very close to the injector (fuel injection nozzle). Such an arrangement involves the disadvantage that only an extremely small operational point-specific ignition window is available and therefore a coordination (tuning) of the fuel jet expansion for large characteristic field ranges is critical. Consequently, the fuel injectors used in such systems have to be manufactured with high precision; even small tolerance deviations or changes in the injector during an extended operation may lead to disadvantageous boundary conditions for the ignition.
Thus, conditions of a reliable ignition in the stratified-charge operation may be ensured only by means of a precise geometrical arrangement of the ignition device and the fuel jet. Consequently, the known methods belonging to this category are designed without a definite and intensive charge motion. In the homogeneous operation, on the other hand, it is precisely such a charge motion that would be needed for improving the homogenization of the fuel/air mixture. The result is a loss of power and an increase of the fuel consumption which, in turn, involves a correspondingly increased release of pollutants into the atmosphere.
It is a further disadvantage of this process that because of the direct exposure of the spark plugs to the fuel jet, an increased spark plug wear occurs, resulting in a shortened service life of the spark plugs.
According to the "wall-guided" process, in the stratified-charge operation the fuel jet is deflected towards the ignition device by the wall of the combustion chamber. The resulting intensive charge motion has an enhancing effect. This method avoids a direct impingement of fuel on the spark plugs. Tolerance deviations and the operational condition of the fuel injectors are less critical than in the earlier-discussed jet-guided process. It is, however, a disadvantage of the wall-guided process that the fuel impinges on the combustion chamber wall during a direct injection into the cylinder chamber so that in certain operational conditions an incomplete combustion occurs which results in an increased emission of uncombusted hydrocarbons and soot. This process has been utilized heretofore with intake-side fuel injectors and is based on the generation of a rolling motion of the charge, particularly designed with respect to direction and sense of rotation. This type of charge motion is achieved by providing steep, upstanding intake ports, such as disclosed in European Patent No. 0 558 072. Such an arrangement, however, requires a correspondingly greater structural height of the engine. According to another solution, the desired motional configuration of the charge is obtained by a special form of the intake port or by the geometry at the seating zone of the intake valve, as disclosed in European Patent No. 0 463 613. Such a solution, however, has disadvantageous effects on the quality of flow of the intake system and thus on the fullload operation of the engine.
The intensive charge motion required in the stratified-charge operation is, however, disadvantageous in the homogeneous operation because of the resulting increased combustion noises and increased wall heat losses.